The major objective of this research project is the elucidation of the mechanism(s) by which kaurene synthetase, and other diterpene cyclases, effect the cyclization of geranylgeranyl and copalyl pyrophosphates to polycyclic diterpenes in soluble enzyme preparations. Relevant experimental criteria are stereochemistry, isotope effects, and the consequences of structural alterations of the substrates. The results are significant in relation to the biosynthesis of the plant growth-regulating gibberellin family and, in general, to terpene and sterol biosynthesis. It is also hoped that the stereospecificities of diterpene cyclases from the same plant source (Ricinus communis) may reveal evolutionary relationships of these apparently similar proteins. The preparation of enzyme extracts possessing terpene cyclase activity from certain marine sources is also under investigation. The total synthesis of moenocinol has been undertaken in order to confirm its unusual sesterpene structure and as a prelude to studies on its biosynthesis. Since carbonium ion rearrangements play a key role in terpene biosynthesis, the structure and stability of certain non-classical carbonium ions generated from cyclopropane ring participation in rigid ring structures has been investigated.